feat(host/steam): M2 — virtual Steam Deck as a wired PadBackend (Linux)

Make the virtual hid-steam device a selectable per-session host gamepad,
end-to-end on Linux: PUNKTFUNK_GAMEPAD=steamdeck now builds a
SteamControllerManager that creates a /dev/uhid 28DE:1205 Deck, enters
gamepad_mode, and feeds the byte-exact Deck report (M1).

- inject/linux/steam_controller.rs: SteamControllerManager / SteamDeckPad,
  mirroring dualsense.rs (open/create2, GET/SET_REPORT pump, heartbeat, RAII
  destroy). Two Steam-specific quirks beyond the DualSense path:
    * gamepad_mode entry — best-effort `lizard_mode=0` via sysfs, plus a b9.6
      creation pulse (MODE_ENTER) so steam_do_deck_input_event stops
      early-returning, plus an anti-toggle guard (MENU_HOLD_CAP) so a long
      in-game Start-hold can't flip gamepad_mode back off.
    * UHID_SET_REPORT answered err=0 (DualSense omits it; the kernel stalls
      ~5s/cmd otherwise); the 0xEB rumble report parsed onto the 0xCA plane.
- core config.rs: GamepadPref::SteamDeck (wire byte 6) + SteamController
  (byte 5, reserved — folds to Xbox360 until its backend lands); from_u8 /
  from_name / as_str. Forward-compatible (unknown byte -> Auto); the C-ABI
  PUNKTFUNK_GAMEPAD_* constants stay M3, so no generated-header drift.
- punktfunk1.rs: PadBackend::SteamDeck variant + select / handle / apply_rich
  / pump / heartbeat arms; pick_gamepad Linux arm.

On-box: an #[ignore]d backend test (backend_binds_and_input_flows) drives the
real SteamDeckPad — it binds hid-steam (gamepad + IMU evdevs), enters gamepad
mode, BTN_A reaches the evdev, and the device tears down on drop. Workspace
clippy/fmt/test green. Not pushed. Next: M3 (protocol/ABI wire) + M4 (client
capture).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-core/src/config.rs

@@ -137,8 +137,9 @@ impl CompositorPref {
 /// host decide (its `PUNKTFUNK_GAMEPAD` env var, else X-Box 360). A concrete preference is
 /// honored only if that backend is available on the host (DualSense / DualShock 4 need Linux UHID);
 /// otherwise the host falls back and reports the real choice in `Welcome`. The wire form is a single
-/// byte (`0 = Auto`, `1 = Xbox360`, `2 = DualSense`, `3 = XboxOne`, `4 = DualShock4`), appended to
-/// `Hello`/`Welcome` — older peers simply omit/ignore it (an unknown byte degrades to `Auto`).
+/// byte (`0 = Auto`, `1 = Xbox360`, `2 = DualSense`, `3 = XboxOne`, `4 = DualShock4`,
+/// `5 = SteamController`, `6 = SteamDeck`), appended to `Hello`/`Welcome` — older peers simply
+/// omit/ignore it (an unknown byte degrades to `Auto`).
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
 pub enum GamepadPref {
     /// Let the host pick (its `PUNKTFUNK_GAMEPAD` env var, else X-Box 360).
@@ -155,10 +156,19 @@ pub enum GamepadPref {
     /// UHID DualShock 4 (kernel `hid-playstation`, ≥ 6.2) — lightbar, touchpad, motion, rumble. Like
     /// `DualSense` minus adaptive triggers / player LEDs / mute. Needs Linux UHID on the host.
     DualShock4,
+    /// UHID classic Steam Controller (Valve `28DE:1102`, kernel `hid-steam`) — dual trackpads, gyro,
+    /// two grip paddles, trackpad-only haptics. Needs Linux UHID. *(Reserved; its backend is not yet
+    /// built — currently folds to `Xbox360`; the Deck identity below is the implemented one.)*
+    SteamController,
+    /// UHID Steam Deck controller (Valve `28DE:1205`, kernel `hid-steam`) — full Deck gamepad incl.
+    /// the four back grips (L4/L5/R4/R5), a right trackpad, and the IMU; re-grabbed by Steam Input
+    /// with native glyphs when Steam runs on the host. Needs Linux UHID.
+    SteamDeck,
 }
 
 impl GamepadPref {
-    /// Wire byte. `0 = Auto`, `1 = Xbox360`, `2 = DualSense`, `3 = XboxOne`, `4 = DualShock4`.
+    /// Wire byte. `0 = Auto`, `1 = Xbox360`, `2 = DualSense`, `3 = XboxOne`, `4 = DualShock4`,
+    /// `5 = SteamController`, `6 = SteamDeck`.
     pub const fn to_u8(self) -> u8 {
         match self {
             GamepadPref::Auto => 0,
@@ -166,6 +176,8 @@ impl GamepadPref {
             GamepadPref::DualSense => 2,
             GamepadPref::XboxOne => 3,
             GamepadPref::DualShock4 => 4,
+            GamepadPref::SteamController => 5,
+            GamepadPref::SteamDeck => 6,
         }
     }
 
@@ -177,6 +189,8 @@ impl GamepadPref {
             2 => GamepadPref::DualSense,
             3 => GamepadPref::XboxOne,
             4 => GamepadPref::DualShock4,
+            5 => GamepadPref::SteamController,
+            6 => GamepadPref::SteamDeck,
             _ => GamepadPref::Auto,
         }
     }
@@ -192,12 +206,14 @@ impl GamepadPref {
                 GamepadPref::XboxOne
             }
             "dualshock4" | "dualshock" | "ds4" | "ps4" => GamepadPref::DualShock4,
+            "steamdeck" | "steam-deck" | "deck" => GamepadPref::SteamDeck,
+            "steamcontroller" | "steam-controller" | "steamcon" => GamepadPref::SteamController,
             _ => return None,
         })
     }
 
     /// Canonical lowercase identifier (`"auto"`, `"xbox360"`, `"dualsense"`, `"xboxone"`,
-    /// `"dualshock4"`).
+    /// `"dualshock4"`, `"steamcontroller"`, `"steamdeck"`).
     pub fn as_str(self) -> &'static str {
         match self {
             GamepadPref::Auto => "auto",
@@ -205,6 +221,8 @@ impl GamepadPref {
             GamepadPref::DualSense => "dualsense",
             GamepadPref::XboxOne => "xboxone",
             GamepadPref::DualShock4 => "dualshock4",
+            GamepadPref::SteamController => "steamcontroller",
+            GamepadPref::SteamDeck => "steamdeck",
         }
     }
 }